A: Working with a colleague at the Wall Street Journal, we set out to ask that exact question. The answer: At least 15.3 million Americans lived within a mile of a well that has been drilled since 2000. That is more people than live in Michigan or New York City.

Here's how we did it: we obtained data documenting the locations and drilling dates of more than 2.3 million wells in 11 states from DrillingInfo, a data provider to the oil industry, as well as the Ohio Department of Natural Resources. We figured out which wells were drilled after 2000 -- and our data is good through the end of 2012, although a small percentage of the wells were drilled in 2013. We discarded well locations where we couldn't determine when it had been drilled.

A: It may be -- eventually. There certainly are plenty of shales around the world. These include include Russia’s giant Bazhenov Shale, the Kimmeridgian Shales in the North Sea and the Qusaiba “hot shale” in Saudi Arabia.

According to report commissioned by the U.S. Energy Information, there are approximately ten times more oil and gas resources in shales outside the U.S. than inside the U.S. Some exploration and development has begun in South Africa’s Karoo Shale, the Vaca Muerta Shale in Argentina as well as others in China, the United Kingdom, and Poland. The Eagle Ford Shale in South Texas is believed to extend beneath the international boundary into Mexico.

Nonetheless, the vast majority of wells drilled into shales and fracked have been in the U.S. and Canada. There are several reasons for this: Fracking was developed in the United States, and North America has a much larger specialized service industry (rigs, pressure pumping units, etc.) than anywhere else in the world. More wells have been drilled and there is a much better geological record in the U.S. than elsewhere. That makes it easier to find shales.

Moreover, the mineral rights are usually owned by landowners in the United States, creating a financial incentive for the landowner to sign a lease with an oil and gas company. Outside of North America, most of the mineral rights are owned by the federal governments. This has caused tension between local communities, who do not stand to benefit directly from extraction activities, and the oil and gas companies.

A: Fracking was first applied to shales that contain natural gas, then later used in shales that contain crude oil and natural gas liquids, such as ethane and butane. It works in both, and it’s worth noting that a lot of wells that are fracked tend to produce a mix of oil and gas.

Companies migrated away from gassy shales when the price of natural gas started falling in 2010. They moved towards the Bakken Shale in North Dakota and the Eagle Ford in Texas because these produced mostly oil and the price of oil remained high while gas fell.

The largest shale, by oil and gas output, in the world is the Marcellus Shale, which stretches from western New York State, covers the western half of Pennsylvania and all of West Virginia, is the largest. In April 2014, according to the Energy Information Administration’s Drilling Productivity Report, Marcellus wells produced 40,131 barrels of oil and 14.8 billion cubic feet of natural gas every day – the energy equivalent of 2.5 million barrels of oil a day.

Texas’ Eagle Ford Shale came in a close second with an oil-and-gas equivalent of 2.45 million barrels a day – split 45%:55% gas to oil. In West Texas, the Permian Basin had 2.33 million barrels a day, mostly oil. The Permian is a collection of shales and other rocks stacked vertically atop one another.

The Bakken Shale in North Dakota was a distant fourth with the oil-and-gas equivalent of 1.28 million barrels a day. The Bakken receives a lot of attention because most of its output is oil and it is taking place in the sparsely populated western half of North Dakota. The rapid population growth has led to many news articles about apartment shortages and fast-food restaurants unable to keep workers.